The present invention relates to a method for forming a polymer film having controlled physical characteristics such as thickness and solidification rates from heated solutions with variable solvent concentrations and articles of manufacture produced thereby.
This invention relates to a new use of a spray forming process which has been developed at the Idaho National Engineering Laboratory (INEL), the process is now referred to as the Controlled Aspiration Process (CAP). The CAP process is set forth in some detail in U.S. Pat. No. 4,919,853 issued to Alvarez and Watson, Apr. 24, 1990, for Apparatus and Method For Spraying Liquid Materials, the disclosure of which is herein incorporated by reference. The nozzle herein identified as a converging/diverging nozzle is the nozzle disclosed in the '853 patent.
The CAP process of spray forming metals aspirates a molten metal into the throat of a converging/diverging gas nozzle, where the liquid is nebulized into a directed spray of rapidly cooling droplets. The gas flow (usually an inert gas such as argon) accelerates the droplets toward the substrate, against which the droplets impact before completely solidifying. Rapid cooling occurs in flight by a variety of thermodynamic mechanisms including convection and radiation as well as by convection and conduction upon arrival at the substrate surface. Temperature control in the CAP process may be achieved by varying one or more of the carrier gas, the nozzle, the temperature of the material being sprayed in its reservoir or tundish or the substrate receiving the nebulized droplets. The droplets deposition and solidification can be closely controlled in the CAP process by temperature and pressure control to produce a polymer film with predetermined physical and chemical properties.
By substituting various polymers for metal in some cases rather unexpected and surprising results have been obtained. Rapid solidification of the nebulized polymer droplets in the plume produced by the CAP process has in some cases resulted in production of asymmetrical polymer membranes having superior separation properties.
The CAP process is beneficial because with it polymers which cannot readily be dissolved in solvents can be sprayed and polymer systems normally sprayed with solvents can be sprayed without. This is an important environmental benefit due to the severe reduction in VOC (volatile organic compound) emissions associated with spraying paints and enamels. The CAP process can be manipulated to control the deposition and solidification of any polymer film. Due to the uniform droplet size distribution in the plume from the CAP process, consistent film thickness and spatially uniform film physical and/or chemical properties are achieved.
The CAP coating capabilities provide precise control of the spray forming process and the deposits which result therefrom have relatively low porosity due to the low gas pressure used in the CAP process resulting in a low droplet velocity. Particularly, using pressures in the range of about 14-26 psi absolute, normalized to one atmosphere, low droplet velocity results in gentle droplet impact conditions at the substrate surface and like gas entrapment.
Most separation processes for solutes and suspended materials are performed using either membranes or filters. The main distinction between a filter and a membrane is that filter pores tend to be arranged adjacent to each other, forming an array of holes. Porosity is not well-connected in a membrane, so atomic/molecular traverses must occur either along very tortuous pathways or by diffusion, resulting in rather low permeabilities and slow speeds of migration. Consequently, filters are normally used to remove relatively large objects from a liquid medium, such as particulates and suspended solids, while membranes are often better suited for separating chemicals in solutions. This distinction is no longer precise, however, when microfilters and high permeability membranes are discussed, where differences can be quite minor.
At the Idaho National Engineering Laboratory (INEL), there has been substantial work on polyphosphazene membranes useful in harsh environments, both chemical and physical, the work has been reported in the Allen et al. U.S. Pat. No. 4,749,489 issued Jun. 7, 1988, the disclosure of which is incorporated herein by reference. The polyphosphazenes are useful as semipermeable membranes because they may be used for separations at temperatures in excess of 179.degree. C. and at pressures in excess of 1.8.times.10.sup.6 Pa. In addition, the membranes formed from the polyphosphazene as reported in the '489 patent, may be useful in harsh chemical conditions. There are other polymer systems which can be sprayed with the CAP process that can exist in harsh environments such as Teflon and other fluorochloro substituted polyethylenes. Although polyphosphazene is used to explain the operation of the CAP process it should be remembered that the process is a means to manipulate various polymers and systems of polymers and is not limited to any one polymer or system of polymers.
Polyphosphazenes are a polymeric material having an inorganic backbone comprising alternating nitrogen and phosphorus atoms which are in turn connected by alternating double bonds. Three basic polymer types can be prepared: linear, cyclolinear and cyclomatrix, as illustrated below: ##STR1##
A base unsaturated phosphazene polymer has two chlorine atoms attached to each phosphorous. These chlorine can be substituted by organic groups such as hydroxyl (R--OH), primary amine (R--NH.sub.2), secondary amines (R.sub.2 --NH) or mercapto (R--SH) groups. In general, the polyphosphazene polymers for use in forming the membranes of the subject invention may be prepared using one of Allcock's procedures. H. R. Allcock, "Phosphorous-Nitrogen Compounds--Cyclic, Linear, and High Polymeric Systems," Academic Press, New York 1972, Chapter 16. Substituted polyphosphazene polymers may be derived from the cyclotrimer hexahalocyclotriphosphazene (usually the hexachloro-trimer is used). When heated to about 250.degree. C., the trimer (hexachloro) polymerizes by a ring cleavage mechanism to form the linear polydichlorophosphazene. A variety of side groups can be attached to the polymer by nucleophilic replacement of the halide side groups with alkoxy-, aryloxyamino or thio groups. Alkyl and aromatic groups can be attached to the polymer by the reaction of polydihalophosphazene with organometallic nucleophiles. In these reactions, the preferred substrate is polydifluorophosphazene. The cyclolinear and cyclomatrix polymers may be prepared by reacting the cyclic trimer with a difunctional monomer. The type of polymer obtained is dependent on the mole ratios of the reactants used and available reactive sites on the trimer. Such reactions are taught by Allcock and are well known in the art. H. R. Allcock, "Phosphorus-Nitrogen Compounds-Cyclic, Linear, and High Polymeric Systems," Academic Press, New York 1972, Chapter 16, which teachings are incorporated herein in their entirety.
From the examples and test results reported in the '489 patent, it is apparent that various membranes can be prepared from phosphazene polymers, which can be selectively synthesized to have the capability of separating only desired solutes from a fluid, whether gas or liquid. This capability is achieved through the substitution of various select groups on the phosphorus atom in the polymer structure. The resulting polyphosphazene is highly heat resistant as well as being chemically resistant. Thus, harsh environments, such as high or low pH, organic solvents and high temperatures will present only minimal deterrents to the use of polyphosphazene membranes in separation techniques.
The CAP process is uniquely capable of spray forming thin layers of synthetic organic resins with superior properties. The polymer layers thus formed can be permanently bonded, where desired, to the base material by chemical reaction or by surface interlinkage. When polymer layers have been deposited by the CAP process, there is nearly complete consolidation of the incident polymer droplet at the substrate for a thickness of about 1 micron and nearly theoretically dense structures are formed. In some cases, a layer can be uniformly dense throughout, while in other cases, deposits after about 1 micron are more porous, thereby producing an asymmetrical membrane having (in the polyphosphazene hereinafter disclosed) vastly superior selectivity over membranes made by knife casting, spin casting or other well known methods.